1. Field of the Invention
This invention is directed to a method for making a printing plate and to a printing plate made according to such a method. More particularly, this invention is directed to a process in which a printing plate precursor is provided which comprises a topmost etchable first layer and a second layer located below the first layer, wherein the first and second layers have different affinities for at least one printing liquid. The first layer is imagewise etched by kinetic energy obtained from the rapid vaporization of liquid droplets. The vaporization is achieved by impinging the liquid droplets with laser energy in close proximity to the topmost first layer.
2. Background Information
The art of lithographic printing is based upon the immiscibility of oil and water, wherein the oily material or ink is preferentially retained by the image area and the water or fountain solution is preferentially retained by the non-image area. When a suitably prepared surface is moistened with water and an ink is then applied, the background or non-image area retains the water and repels the ink while the image area accepts the ink and repels the water. The ink on the image area is then transferred to the surface of a material upon which the image is to be reproduced, such as paper, cloth and the like. Commonly the ink is transferred to an intermediate material called the blanket which in turn transfers the ink to the surface of the material upon which the image is to be reproduced.
A very widely used type of lithographic printing plate has a light-sensitive coating applied to an aluminum base support. The coating may respond to light by having the portion which is exposed become soluble so that it is removed in the developing process. Such a plate is referred to as positive-working. Conversely, when that portion of the coating which is exposed becomes hardened, the plate is referred to as negative-working. In both instances the image area remaining is ink-receptive or oleophilic and the non-image area or background is water-receptive or hydrophilic. The differentiation between image and non-image areas is made in the exposure process where a film is applied to the plate with a vacuum to insure good contact. The plate is then exposed to a light source, a portion of which is composed of UV radiation. In the instance where a positive plate is used, the area on the film that corresponds to the image on the plate is opaque so that no light will strike the plate, whereas the area on the film that corresponds to the non-image area is clear and permits the transmission of light to the coating which then becomes more soluble and is removed. In the case of a negative plate the converse is true. The area on the film corresponding to the image area is clear while the non-image area is opaque. The coating under the clear area of film is hardened by the action of light while the area not struck by light is removed. The light-hardened surface of a negative plate is therefore oleophilic and will accept ink while the non-image area which has had the coating removed through the action of a developer is desensitized and is therefore hydrophilic.
Lithographic plates may be divided into classes based upon their affinity for printing ink. Those which require dampening water which is fed to the non-image areas of the plate, forms a water film and acts as an ink-repellant layer; this is the so-called fount solution. Those which require no fount solution are called driographs or water-less lithographic plates. Most lithographic plates at present in use are of the first type and require a fount-solution during printing. However, lithographic plates of this type suffer from a number of disadvantages. Some of these are:
Adjustment of the proper ink-water balance during press operation is difficult and requires great experience. If the correct ink-water balance is not achieved scumming is occasioned when the printed ink image extends into the non-image areas thereby ruining the printed image.
Adjustment of the ink-water balance at start-up or re-start up is particularly difficult and can not be stabilized until a large number of sheets have been printed, thus incurring waste.
The ink tends to become emulsified which leads to poor adherence of the ink onto the plate which causes problems in color reproduction and in dot reproduction.
The printing press has to be provided with a dampening system, thus increasing its size and complexity. These dampening solutions contain volatile organic compounds.
The plate care chemistry and fount solutions require careful control and selection. In addition, plate cleaners contain significant levels of solvent which is not desirable.
However, with water-less plates in which the ink-releasing layer is, for example, a cured silicone layer there is no scumming and clearer images can be produced. Very often water-less plates comprise a base material, for example aluminum plate, on which a photosensitive layer is coated, on this photosensitive layer there is coated a silicone layer. After imagewise exposure and development in which selected areas of the photosensitive composition are altered, the overlying silicone layer is removed and the plate is inked up. The ink adheres only to those areas of the plate not covered by the silicone remaining after development. Thus the plate can be printed without the need to use a fount solution. In practice it is difficult and costly to formulate and manufacture the silicone layer composition with sufficient adhesion to the photosensitive composition in these multilayer assemblies. Thus the only commercially available water-less lithographic plates are expensive and of complex design.
There exists in patent literature water-less lithographic plate designs which do not exhibit these disadvantages. These inventions disclose photosensitive water-less lithographic plate precursors comprising a support with an oleophilic surface and a single layer, photosensitive, ink-releasing composition such that imagewise exposure causes changes in developer solubility of the composition where development produces an ink accepting image pattern on the uncovered support surface and an ink-releasing non-image area corresponding to unremoved composition.
There are numerous known methods for creating image and non-image areas. Some methods rely on the differential solubility of exposed and non-exposed areas in a developer; others use incident radiation to break covalent bonds of radiation sensitive formulations or to ablate a layer of material.
Lithography and offset printing methods have long been combined in a compatible marriage of great convenience for the printing industry for economical, high speed, high quality image duplicating in small runs and large. Known art available to the industry for image transfer to a lithographic plate is voluminous but dominated by the photographic process wherein a hydrophilic plate is treated with a photosensitive coating, exposed via a film image and developed to produce a printable, oleophilic image on the plate.
While preparing lithographic plates by photographic image transfer is relatively efficient and efficacious, it is a multi-step, indirect process of constrained flexibility. Typically, a photographically presensitized (PS) plate is prepared from a hydrophilic surface-treated aluminum. A positive or negative film image of an original hard copy is prepared and the PS plate exposed to the film image, developed, washed and made ready for print operations. Any desired changes in the film image must be made by first changing the original hard copy and repeating the photographic process; hence, the constrained flexibility. As sophisticated and useful as it is to prepare plates by photographic image transfer, the need for a lithographic plate fabricating process that obviates the above problems associated with the photographic process has long been recognized.
Clearly, it would be highly beneficial to the printing industry to directly produce a quality printable image on a plate without proceeding through a multi-step photographic process. It would also be highly efficacious if a process were developed whereby changes could be made in an original image in some predetermined manner without incurring the need to correct hard copy and repeat the photography, particularly if those changes could be made xe2x80x9con line.xe2x80x9d Consistent with these goals, artisans in the field of lithographic plate production have recently come to direct their efforts toward the development of a means to integrate digitally controlled image-making technology, i.e., the ubiquitous personal computer, with a means to directly convey the digital image onto a lithographic plate that will be usable for large production runs (100,000 or more copies).
Image forming by digital computer aided design of graphical material or text is well known. Electronically derived images of words or graphics presented on the CRT of a digital computer system can be edited and converted to final hard copy by direct printing with impact printers, laser printers or ink jet printers. This manner of printing or producing hard copy is extremely flexible and useful when print runs of no more than a few thousand are required but the print process is not feasible for large runs measured in the tens or hundreds of thousands of pieces. For large runs, printing by lithographic plate is still the preferred process with such plates prepared by the process of photographic image transfer.
As disclosed, for example, at col. 2, line 21 to col. 3, line 10 of co-assigned U.S. Pat. No. 5,908,705 and the references cited therein, and U.S. Pat. No. 5,339,737 and the references cited therein, lasers and their amenability to digital control have stimulated a substantial effort in the development of laser-based imaging systems. Early examples utilized lasers to etch away material from a plate blank to form an intaglio or letterpress pattern. This approach was later extended to production of lithographic plates, e.g., by removal of a hydrophilic surface to reveal oleophilic underlayers. These systems generally require high-power lasers which are expensive and slow.
A second approach to laser imaging involves the use of thermal-transfer materials. With these systems, a polymer sheet transparent to the radiation emitted by the laser is coated with a transferable material. During operation the transfer side of this construction is brought into contact with an acceptor sheet, and the transfer material is selectively irradiated through the transparent layer. Irradiation causes the transfer material to adhere preferentially to the acceptor sheet. The transfer and acceptor materials exhibit different affinities for fountain solution and/or ink, so that removal of the transparent layer together with non-irradiated transfer material leaves a suitably imaged, finished plate. Typically, the transfer material is oleophilic and the acceptor material hydrophilic. Plates produced with transfer-type systems tend to exhibit short useful lifetimes due to the limited amount of material that can effectively be transferred. In addition, because the transfer process involves melting and resolidification of material, image quality tends to be visibly poorer than that obtainable with other methods.
Lasers have also be used to expose a photosensitive blank for traditional chemical processing. In an alternative to this approach, a laser has been employed to selectively remove, in an imagewise pattern, an opaque coating that overlies a photosensitive plate blank. The plate is then exposed to a source of radiation with the unremoved material acting as a mask that prevents radiation from reaching underlying portions of the plate. Either of these imaging techniques requires the cumbersome chemical processing associated with traditional, non-digital platemaking.
Lithographic printing plates suitable for digitally controlled imaging by means of laser devices have also been disclosed in the prior art. Here, laser output ablates one or more plate layers, resulting in an imagewise pattern of features on the plate. Laser output passes through at least one discreet layer and imagewise ablates one or more underlying layer. The image features produced exhibit an affinity for ink or an ink-abhesive fluid that differs from that of unexposed areas. The ablatable material used to describe the image is deposited as an intractable, infusible, IR absorptive conductive polymer under an IR transparent polymer film. As a consequence, the process of preparing the plate is complicated and the image produced by the ablated polymer on the plate does not yield sharp and distinct printed copy.
Flexographic printing plates are also well known to those skilled in the art. Flexographic printing typically involves one of three different types of image carriers:
Rubber plates, in which a negative of the desired image is placed on a metal alloy coated with a light sensitive acid resist. Upon exposure, the exposed resist areas harden and become insoluble, but the unexposed areas remain soluble and are washed away. An etchant is applied to the surface, thereby engraving the areas unprotected by the hardened resist, and resulting in a metallic relief plate. A mold is then made of the relief plate, and a rubber sheet is pressed into the mold to obtain a rubber relief plate.
Photopolymer plates, in which a photographic negative of the desired image is placed on a photopolymeric material which is then exposed to UV radiation, thereby hardening the photopolymer in the exposed areas. The unhardened areas of the photopolymer are removed via washing, leaving the image areas in relief.
Design rolls, in which a layer of vulcanized rubber is applied to the surface of a plate cylinder, and the desired image is engraved thereupon using a high energy laser, which atomizes rubber in the non-image areas, thus leaving the image areas in relief. The height of the image above the floor of the cylinder can be varied in the engraving process, depending upon the level of relief desired.
The use of laser radiation to cut or ablate materials in medical and dental applications is well known. For example, U.S. Pat. Nos. 5,020,995; 5,194,005; and 5,762,501 disclose the use of laser radiation having a selected wavelength to cut, by vaporization, dentin, tooth enamel, gum tissue, vascularized tissue, bone, metal fillings and the like. In addition, U.S. Pat. Nos. 5,741,247 and 5,785,521 disclose the use of a laser in medical and dental applications for accurate cutting of hard and soft tissue and other materials. More particularly U.S. Pat. No. 5,741,247 discloses an apparatus in which laser energy is used to vaporize or explode atomized fluid particles in the vicinity of the target area. The explosive forces released from the vaporized fluid particles impart mechanical cutting forces onto the target.
It is one object of this invention to provide a method of preparing a printing plate, in which a printing plate precursor is directly imaged by imagewise etching the precursor using kinetic energy derived from the rapid vaporization of liquid droplets which have absorbed laser energy. It is another object of this invention to provide a printing plate prepared using the method of this invention. This invention advantageously permits the desired image to be etched directly upon the printing plate precursor, thereby avoiding the need for films, masks, wet chemistry or exposure techniques. It is one feature of this invention that the imaging may be accomplished via a digital system which controls the placement of the laser radiation and targeting of the kinetic energy to selected portions of the etchable material portion of the precursor. It is another feature of this invention that no pre-exposure, post-exposure, or post-imaging chemical treatments are required to xe2x80x9cdevelopxe2x80x9d the desired image. It is another advantage of this invention that it is xe2x80x9cwhite light safe,xe2x80x9d i.e., since the method does not depend upon a photochemical change to occur in the precursor to obtain the desired image, the need of protecting the precursor from xe2x80x9cwhitexe2x80x9d light (e.g. sunlight) is obviated. It is another feature of this invention that the thermal stability of the resulting printing plate is not compromised due to direct response of the imageable portion of the precursor to the laser. This invention also advantageously results in a low amount of residue material (typically dust) remaining on the plate surface after ablative imaging, thereby avoiding possible damage to the imaging and printing equipment due to the presence of such dust. It is another feature of this invention that the laser imaging and plate system described herein is less expensive than conventional laser thermal imaging and plate systems. Other objects, features and advantages of this invention will be readily apparent to those skilled in the art.
This invention is directed to a method of producing a printing plate comprising:
(a) providing a printing plate precursor comprising a topmost etchable first layer and a second layer located below the first layer, wherein the first and second layers have different affinities for at least one printing liquid;
(b) imagewise providing atomized fluid particles in an interactive zone located above the top surface of the first layer; and
(c) directing laser energy into the interactive zone, wherein the laser energy has a wavelength which is substantially absorbed by the atomized fluid particles in the interaction zone, and the absorption of the laser energy causes the atomized fluid particles to imagewise impart kinetic energy to and etch the first layer.
This invention is also directed to printing plates prepared by the above-described method. Printing plates which may be prepared in accordance with this invention include waterless plates, positive-and negative-working plates, and flexographic plates.